The present invention relates to electrochemical cells having a mechanism for protecting against damage from overcharge. More specifically, the invention relates to cells in which sulfur or a similarly acting species is oxidized during overcharge at a positive electrode and then shuttles to a negative electrode where it is reduced.
Damage from overcharge presents a significant problem for many secondary batteries. Normal recharging is intended to be carried out until the cell on recharge reaches a defined voltage. If for any reason this voltage is exceeded, overcharge may result. Overcharge can cause various undesirable reactions such as destruction of the cell electrolyte, corrosion of current collectors, degradation of cell separators, and irreversible damage to the positive or negative electrode. Any one of these conditions can lead to the destruction of the cell. Further, overcharge can create unsafe conditions such as cell venting due to electrolyte gassing.
The problem may be especially pronounced when a plurality of electrochemical cells are coupled together in series (e.g., to form a battery or battery pack). Typically, the individual cells will possess at least slightly varying capacities--reflected as the maximum number of coulombs (or ampere-hours) that the cell accept without overcharging. Those cells having lower capacities will become fully charged before other cells having higher capacities. If charging continues after the lower capacity cells reach full charge, the lower capacity cells will overcharge and possibly be damaged.
Various approaches to overcharge protection have been employed. In one approach, a protective additive is provided to the cell. Such additives undergo "parasitic" reactions at cell potentials above the cell's full charge potential but below a destructively high cell potential. Thus, suitable additives are chosen based upon characteristic voltages at which they are oxidized and reduced. If an electrode voltage reaches the additive's characteristic oxidation or reduction potential, the additive begins to react and continues to react until the cell potential recedes to a safe level.
One widely-used class of protective additives is based on the organometallic ferrocene compounds. A given ferrocene may be oxidized at a voltage of about 3 volts (versus a lithium negative electrode) for example. Consider a ferrocene-protected lithium-iron sulfide cell having a normal full charge cell potential of about 1.8 volts. When during charge of such cell, all positive electrode material has been fully oxidized, the cell voltage increases beyond 1.8 volts towards 3 volts. When the cell voltage reaches 3 volts, the ferrocene additive begins to react. Specifically, it is oxidized at the positive electrode. The oxidized compound then travels to the negative electrode where it is reduced. The reduced compound then shuttles back to the positive electrode to again be oxidized. In this manner, the ferrocene provides a shuttle redox mechanism, thereby protecting the cell from attaining too high of a voltage.
As most widely-used redox shuttle additives, like ferrocenes, are cyclic organic compounds, they eventually degrade under the harsh cell environment of a rechargeable alkali-metal cell. Thus, the protection they provide eventually decreases during the cell's life.
Further, because such redox additives invariably react at a given potential, they may sometimes impede normal cell charging. For example, during rapid charging, the cell voltage may slightly exceed the point at which the additive reacts, even though the cell has not been charged to full capacity. When this occurs, the charging current shunts to the additive's redox reaction and away from the desired charging reaction.
At least one inorganic overcharge protective additive has been employed. In the article "Overcharge Protection in Li-Alloy/Metal Disulfide Cells" by L. Redey, Proceedings--Electrochemical Society (Proc. Jt. Int. Symp. Molten Salts), 87-7, pages 631-636, (1987), molten-salt-electrolyte lithium-alloy/metal-sulfide cells employing lithium sulfide as an overcharge protection agent are described. In these cells, the lithium sulfide is soluble in molten electrolyte. During overcharge, it reacts at the positive electrode to produce a lithium polysulfide of relatively low oxidation state. The polysulfide then shuttles to the negative electrode where is it reduced back to sulfide on the lithium alloy. The sulfide additive used in this redox shuttle is more robust in the face of lithium negative electrodes than ferrocenes and other common organic additives. Unfortunately, the protective reaction described in the Redey reference occurs at the rather low voltage of about 1.9 to 2.05 volts. This is lower than the operating voltage for many important cells in use today. Thus, application of the Redey protective mechanism is limited to cells having potentials below about 1.9 volts (e.g., the lithium alloy-iron sulfide molten-salt cell described by Redey).
In view of the above difficulties, what is needed is an improved overcharge protection mechanism employing protective species which (1) resist attack by lithium (or other highly reactive electrode materials), (2) are stable in both the reduced and oxidized states, and (3) do not impede the cell's normal charge and discharge functioning.